The invention relates to optical imaging devices for scanning a field of view and converting an image from the field of view into one or more electrical signals. More particularly, the invention relates to optical scanning devices for scanning an optical image in a field of view. In practice, the image is scanned onto one or more optical detectors.
A conventional optical imaging device, for example an infrared imaging system known as the U.S. Common Module infrared imaging system, is shown in FIG. 1. In this system, light rays 10 from an optical field of view are scanned across and focussed onto a linear detector array 12 via an afocal lens arrangement 14, a scanner 16, and an imaging lens 18. Electrical signals generated by detectors 12 are, for example, amplified by processor 20. Processor 20 may perform other functions as well. Finally, the electrical signal or signals produced by processor 20 are fed, for example, to an array of light emitting diodes (not shown) to produce a visible image corresponding to the infrared image which was scanned. The signals from processor 20 may alternatively be fed to other display devices, they may be sent to a storage device, or they may be further processed, depending upon user requirements.
The conventional system shown in FIG. 1 is a parallel scan system. Scanning is accomplished by focussing bundles of parallel rays of light onto the detectors in array 12. The detectors in array 12 extend in a line perpendicular to the drawing on support 22. As shown in FIG. 1, parallel light rays 10, all of which need not be in the plane of the drawing figure, are first converted into a beam bundle of smaller cross-sectional area by afocal lens arrangement 14. This is done principally for convenience to decrease the size of the detector while maintaining high light-gathering ability. Light rays 10 are then incident on a planar scanning mirror 24 which is caused to reciprocate around axis 26 (axis 26 is perpendicular to the plane of the drawing) as shown by the arrows in FIG. 1.
When mirror 24 is at the position shown in FIG. 1, the light rays 10 are reflected such that they are focussed onto a single detector element of the linear array 12. At this same instant, other light rays from the scene (not shown), which are not parallel to rays 10, are focussed onto other detector elements of array 12 which extends perpendicular to the plane of the drawing (i.e. detector array 12 extends vertically). In this manner, a vertical line from the scene to be observed is imaged onto the detector array.
Now, after mirror 24 rotates to a new position, neither light rays 10 nor the other light rays from the vertical line of the scene are any longer focussed onto detector array 12. The vertical line of the scene is now focussed to one side of array 12. Instead, other parallel beam bundles are focussed onto the array. Consequently, a different vertical line from the scene, horizontally displaced from the first vertical line, is now imaged onto the detector array 12. By continuously rotating mirror 24 through a fixed angle around axis 26, first clockwise and then counterclockwise, the entire scene is sequentially imaged (vertical line-by-vertical line) onto the detector array 12. This type of scaning is described as azimuth scanning. The angular position of mirror 24 around axis 26 (after chosing a reference position of zero) is the azimuth angle.
While in theory the above-described azimuth scanner can scan a complete scene, in practice gaps are created by the finite vertical separation between each detector in the array 12. Thus, the scene is scanned continuously in the horizontal direction but discontinuously in the vertical direction, thereby detecting spaced horizontal lines from the scene.
At the same time mirror 24 is oscillated around axis 26, conventional systems also superimpose a small rocking motion which tilts the mirror 24 around an axis (not shown) which is perpendicular to axis 26 and which is in the plane of the mirror 24. The angular position of mirror 24 around this horizontal axis (after chosing a reference position of zero) is the elevation angle. This superimposed rocking motion varies the elevation angle only slightly, but sufficiently to provide interlace.
The total field of view of the parallel scan system described above is determined by the number, size, and spacing of elements in the linear detector array 12 (which determine the total elevation angle) and by the scan angle (which determines the total azimuth). The scan angle is the maximum angle of rotation of the mirror 24. The ratio of the azimuth angle to the total elevation angle is called the aspect ratio. Typical aspect ratios are 1:1, 4:3, and 2:1.
If larger fields of view are desired in Common Module systems, it is necessary to utilize a longer array of detectors. To maintain resolution, this requires the use of additional detector elements in the array. For example, doubling the number of detector elements provides four times the field of view, if the aspect ratio is kept constant. Unfortunately, while the increased performance attained with more detectors is relatively easily achieved, it is at a much higher cost due to (1) the higher cost of the detector array, (2) the higher cost of the processing electronics, and (3) the larger volume needed to house the imaging device.